An easy way would be to live separately? but I think you know that it's really up to you, you can say no. If you find it too hard, then take measures so that it would either be difficult to do those things(like live separately).

If you really like her and think you can spend the rest of your life with her, maybe you should think about holding things off until marriage. Think about it, you're still young, there's still so much you have yet to experience and one of them should've been sex. If you have too much of it now, in the future it may not be as exciting. Just my 2 cents.